Prior to the advent of fly-by-wire technology, the flight control surfaces on a commercial aircraft were controlled using a complex system of cables and mechanical controls as the primary control path. Such a prior art control system is partially illustrated in FIG. 1. In this type of control system, a pilot's control commands are transmitted from a pair of pilot controls 10 to an individual flight control surface 20 through a series of interconnected cables 12. The cables 12 move one or more valves that control a plurality of hydraulic actuators 15, which in turn move the control surface 20. The cables 12 provide a direct mechanical coupling between the pilot controls 10 and the controlled flight control surface 20. A plurality of strategically placed jam override devices 17 allow continued safe operation of the system in the event a cable becomes jammed.
An electronics bay provides enhanced control functions to the system, by controlling several electrical or electrohydraulic activators. These actuators enhance the control commands input by the pilot based on the flight conditions of the aircraft. Examples of such electro-hydraulic actuators include an outboard aileron lockout mechanism to prevent movement of the aircraft's ailerons at high speed, an aileron droop mechanism 19, which droops the inboard ailerons as a function of flap position, etc. Other servo actuators include a series of autopilot servo actuators 14 which implement auto-pilot commands from an autopilot computer included in the electronics bay.
The prior art control system shown in FIG. 1 has numerous drawbacks that limit its use in modern aircraft. The first drawback with such a system is its high cost of maintenance. The electro-hydraulic actuators, often numbering more than forty on a large aircraft, present a formidable maintenance challenge. Each of these devices is embedded within a complex routing of cables 12 that extends throughout the aircraft and therefore even simple repairs can be labor intensive.
A second drawback of the prior art control system is the difficulty in implementing modern control laws that require a computer to control the aircraft. Since the introduction of these earlier flight control systems, advanced control laws have been developed which, among other things, increase aircraft stability as well as control the speed, rate of climb and descent, banking angles, etc. These control laws are difficult to incorporate into a mechanical control system without a substantial increase in system complexity. Finally, the prior art control system is inherently heavy. In the design of aircraft, it is always desirable to reduce tare weight if it can be done without reducing aircraft safety. Therefore, in order to overcome these and other limitations of prior art flight control systems, modem aircraft are being designed to incorporate fly-by-wire technologies.
In contrast to the mechanical flight control system shown in FIG. 1, a simplified diagram of a fly-by-wire (FBW) system according to the present invention is shown in FIG. 2. In a fly-by-wire system, there is no direct mechanical coupling between the pilot controls 10 and a flight control surface 20. Instead of using cables, a fly-by-wire system includes a set of pilot control transducers 22, which sense the position of the controls 10 and generate electrical signals proportional to the position of the pilot controls 10. The electrical signals are transmitted to an electronics bay 24, where they are combined with other airplane data to produce a flight control surface command that controls the movement of a hydraulic actuator 26 that moves the flight control surfaces 20. A pair of pilot controls 10 are connected by a jam override device 34 so that normally both controllers move together. However, in the event that one of the pilots controls becomes stuck, or jammed, the other pilot control can be freed for use by applying force to the jam override device 34 sufficient to uncouple the two controllers.
Because safety is always a high priority in the aircraft industry, fly-by-wire systems usually include redundant components so that if one component of the system fails, a pilot can still safely control the aircraft. Such redundancy is usually provided on axis-by-axis basis. For example, some prior art fly-by-wire architectures have separate systems that control the movement of the aircraft in each of the roll, pitch and yaw axes.
Each axis control system typically included a primary flight computer and a back-up flight computer that only control movement of the aircraft in the particular axis. If the primary flight computer that controls the roll axis were to fail, the back-up computer would engage to control the roll of the aircraft. Similarly, the pitch and yaw axis systems would each include a primary and back-up flight computer. However, if the back-up computer in an axis channel were to fail, the computers in the other channels could not function to fly the aircraft in that axis. Therefore, a need exists for an integrated fly-by-wire system to reduce the possibility that a failure in one part of the system would leave an aircraft unable to fly safely.
A need also exists for a fly-by-wire system that is divided into a series of independent control channels wherein each control channel within the system is substantially isolated from the other control channels. Thus, a malfunction occurring in one channel does not affect the continued operation of the remaining channels.
Furthermore, a need exists for a fly-by-wire system including a plurality of control channels that are designed such that a failure of one part of a control channel will not affect that control channel's ability to safely fly the aircraft.
Finally, a need exists for a fly-by-wire control system wherein the pilot can fly the aircraft without the assistance of a flight control computer if all the flight control computers included in the system should fail.